If there is an air bubble in a cooling system it can cause the system to not work at all and overheat!
If the bubble gets inside the pump the impeller will just spin in the air, no water will move. If it gets inside the block or rad then the water will not transfer heat, the air will block it. Very dangerous.
You realise I am talking about a large bubble, larger than the diameter of the tubes. The tiny bubbles are not good, but are not dangerous like the bigger bubbles.